A sensitometer is a device which is employed to subject photosensitive material to a graded exposure of light; i.e., the light is passed through a standard means which incrementally attenuates the light in accordance with a predetermined pattern. The light is transmitted through the graded exposure means onto a film or some other form of photosensitive material. The amount of light which passes through the graded exposure means is considered to be known and therefore the sensitivity response is predictable provided the photosensitive material meets some predetermined or standard criteria, or provided the photo developing process conforms to some predetermined or standard criteria. In a manufacturing process, the manufacturer of the film or photo-sensitive material will know what kind of exposure-density (H vs D) relationship is desired for his product. If a sample of the product is exposed to a graded source of light as provided by a sensitometer, data can be gathered which is indicative of the response of the sample to the graded exposure. This data can be plotted or processed to provide an H vs. D curve. If the sample of the product does not respond to the graded exposure of light so that the resultant H vs. D curve approximates the desired curve (within some predetermined limits) then the processing or fabrication of the product is corrected or dealt with in some compensating way (the product could even be scrapped).
It is stated above that in the prior art the quantity of light passing through the graded exposure means was considered to be known. Unfortunately, in the prior art, the quantity of light passing through the graded exposure means was not always consistent and, therefore, was not always known. The foregoing is true for two reasons. First, the light bulb would take more or less time to beat up to its stabilized condition depending upon whether or not the lamp was cool or hot from some prior use. In addition, the transformer which generally was employed to provide power to the lamp would take several cycles of its 60 Hertz frequency to provide an output voltage which had stabilized. Accordingly it was found that the light passing through a graded exposure means varied because of the lack of light and power stabilization. In some embodiments of sensitometers, complex and expensive shutters and voltage stabilization devices are employed to provide consistent characteristics of light source. In the present invention a relatively simple and low cost means is provided to enable the light source to be stabilized with respect to both its power input and element heating.